Prior Art
The conventional bent pipes are prescribed in Japanese Industrial Standard Nos. B2311, B2312, etc. The pipe has been manufactured by attaching straight pipes 3 at weld portions 4 to weld-type fittings 2 (generally known as elbows) as shown in FIG. 2 and manufactured according to the Hamburg bending method as described in "Tekko binran (Steel Manual) VI," 3rd issue, page 179, edited by the Japanese Iron and Steel Institution. However, the configuration of the bent pipe product is limited to a bend part having a bend angle of up to 180.degree. . Hence, the bent pipe product has had problems, such as an increase in welding man days at the time of pipe assembling, an increase in inspection man days for welded parts in accordance therewith, need of a longer construction period, and an increase in construction cost. Therefore, there has been a demand for a bent pipe comprising a bend part and straight parts integral with both ends thereof in order to decrease weld parts. As a proposal for manufacturing such a desirable bent pipe, Japanese Patent Laid-Open Nos. 24878/1974 and 135870/1978 disclose a processing method for an induction bent pipe 1 as shown in FIG. 1. This method has a drawback that the bend part always exhibits a yield starting internal pressure lower than that of the straight part, when the ratio (R/D.sub.0) of bend radius, R, to straight part outside diameter, D.sub.0, is in the range of 1 to 5 and when the ratio (T.sub.0 /D.sub.0) of straight part wall thickness, T.sub.0, to straight part outside diameter, D.sub.0, is in the range of 0.02 to 0.20.